Semiconductor devices often include multiple layers of conductive, insulating and semiconductive layers. Often, the desirable properties of such layers improve with the crystallinity of the layer. For example, the electron mobility and band gap of semiconductive layers improves as the crystallinity of the layer increases. Similarly, the free electron concentration of conductive layers and the electron charge displacement and electron energy recoverability of insulative or dielectric films improves as the crystallinity of these layers increases.
For many years, attempts have been made to grow various monolithic thin films on a foreign substrate such as silicon (Si). To achieve optimal characteristics of the various monolithic layers, however, a monocrystalline film of high crystalline quality is desired. Attempts have been made, for example, to grow various monocrystalline layers on a substrate such as germanium, silicon and various insulators. These attempts have generally been unsuccessful because lattice mismatches between the host crystal and the grown crystal have caused the resulting layer of monocrystalline material to be of low crystalline quality.
If a large area thin film of high quality monocrystalline material was available at low cost, a variety of semiconductor devices could advantageously be fabricated in or using that thin film at a low cost compared to the cost of fabricating such devices beginning with a bulk wafer of semiconductor material or in an epitaxial film of such material on a bulk wafer of semiconductor material. In addition, if a thin film of high quality monocrystalline material could be realized beginning with a bulk wafer such as a silicon wafer, an integrated device structure could be achieved that took advantage of the best properties of both the silicon and the high quality monocrystalline material.
For example, some applications require both high power handling and high frequency performance in an analog or linear circuit, often in combination with digital processing capability. Silicon technology is well-developed for implementing high-power and digital devices such as microprocessors and digital signal processors. Compound semiconductor technology such as Group III-V materials are particularly well suited for high frequency linear applications. However, it has heretofore been difficult if not impossible to integrated these materials in a common monolithic device.
Accordingly, a need exists for a semiconductor structure that provides a high quality monocrystalline film or layer over another monocrystalline material and for a process for making such a structure. Further, a need exists for a semiconductor structure which combines silicon devices and compound semiconductor devices in a common monolithic substrate to provide improved performance in a smaller, lighter, less expensive device.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.